Accelerometers serve as one of the major sensors used in inertial navigation systems as well as various other safety and control systems.
Accurate and reliable accelerometers require great precision and uniform operating results. Prior art accelerometers are generally assembled from a number of components which creates tremendous assembly problems associated with such precision devices. In addition, these accelerometers are typically large and may not be radiation hard.
Although other prior art accelerometers are fabricated utilizing a micromechanical process, no provisions are made for electrically isolating the proof mass from the flexures which is required to accurately and independently drive and sense the resonant frequency of each of the flexures. Additionally, none of these prior art devices have provided reliable means for easily and accurately adjusting the natural resonant frequency of the flexure. Stresses from sources such as impurity doping affect device performance and reliability in the absence of stress relief. Similarly, strain introduced by mechanical bending or thermal stress in the substrate also affects performance due to a lack of strain relief.
Some prior art accelerometers fabricated by a micromechanical process use anodic bonding of a glass substrate containing electrodes to a silicon substrate containing an active silicon device. However, a number of disadvantages associated with silicon-on-glass devices are known. Anodic bonding, typically undertaken at 370 degrees centigrade and using 900 Volts DC, is complicated by the necessity of maintaining the glass and silicon substrates in precise alignment. Such alignment during bonding is additionally complicated by a difference in thermal characteristics between glass and silicon; stress is observed as a result of thermal mismatch. Curling of the silicon substrate from the glass substrate is also commonly observed. Bonding glass and silicon substrates allows foreign matter to lodge in the gap between the proof mass and electrodes or substrate, thus degrading device performance. Further, glass substrates are incompatible with other on-chip circuitry, and are subject to undesirable charge build-up.
Silicon-on-sapphire is also known in the art as a common silicon-on-insulator (SOI) starting material for micromechanical sensors. However, expense and contamination issues have limited the usefulness of this substrate.